1. Field of the Invention
The present invention relates to a porous diaphragm for use in an electrolytic cell, and, more especially, to a porous diaphragm for use in an electrolytic cell to prepare, by electrolysis, high yields of concentrated solutions of alkali metal hydroxides.
2. Description of the Prior Art
It has very long been known to this art to produce chlorine and sodium hydroxide by electrolysis in electrolytic cells fitted with porous diaphragms. And also for a long period of time, such diaphragms were fabricated from asbestos; for the last several years various fluorinated resins were added to or substituted for the asbestos in order to provide diaphragms having improved physical properties. These fluorinated polymers and, in particular, polytetrafluoroethylene, nonetheless have the disadvantage of being difficult to wet with water or aqueous solutions, which hinders or even prevents the percolation of the cell electrolyte through the pores of the diaphragm. This disadvantage was remedied by depositing small amounts of carboxylic acid resins within the pores, as described in French Application No. 80/01843, and its U.S. counterpart application Ser. No. 226,693 now U.S. Pat. No. 4,346,615 as well as French Pat. No. 2,419,985 and its U.S. counterpart, i.e., U.S. Pat. No. 4,222,842. Described therein is the conversion of a porous diaphragm into an ion exchange separator, by the total obstruction of the pores of the diaphragm. The different separators have their own properties; while the diaphragms make it possible to prepare sodium hydroxide in low concentration and containing sodium chloride, the ion exchange separators almost entirely eliminate the presence of chloride in the product hydroxide which may be at a relatively high concentration, but which is obtained in but mediocre yields.